Protective arrangements for prime movers coupled to electric generators



Nov. 10, 1964 B. WOOD 3,156,848 PROTECTIVE ARRANGEMENTS FoR PRIME MOVERSCOUPLED TO ELECTRIC GENERATORS Filed Oct. 2, 1959 Inventor 5561/ 14 000!A tiorneys United States Patent M 3,156,848 PROTECTIVE ARRANGEMENTS FORPREME MOVERS COUPLED T0 ELECTRIC GEN- ERATORS Basil Wood, Esher,England, assignor to Merl & Mc-

Lellan Services Limited, Newcastle-on-Tyne, England, a British companyFiled Get. 2, 1959, Ser. No. 844,606 Claims priority, application GreatBritain, Oct. 6, 1958, 31,805/58 2 (Ilaims. (Cl. 317-19) This inventionrelates to a protective arrangement for a prime mover, such for exampleas a steam or gas or hydraulic turbine or a reciprocating steam orinternal combustion engine, driving an electric generator.

It is known to provide a mechanical shut-down knob, which is operated byhand in emergency (or otherwise) to cut oil the supply of operatingfluid or fuel to the prime mover. Thus, in the case of a steam turbine,it is customary to provide for the control of the steam supply agovernor valve, often operated hydraulically from the governor, andusually also an emergency valve, which is normally latched in the openposition, for example hydraulically, and in such case the mechanicalshut-down knob may conveniently act to unlatch the emergency valve, forexample by rel-easing the hydraulic pressure, and thereby to effectrapid closing of such valve, and at the same time to cause rapid closingof the governor valve by releasing the hydraulic pressure.

It is also known to provide a manually-operated electrical trip buttonto open the main electric circuit-breaker controlling the connectionfrom the generator to the network or other electrical load. It iscustomary to arrange for such trip button also to cause the supply ofoperating fluid or fuel to the prime mover to be cut off, for example bycombining together the electrical trip button and the mechanicalshut-down knob.

It is also usual to provide an electric protective arrangement operativeto trip the main circuit-breaker in. the event of abnormal or faultyconditions arising in the generator or in the associated prime mover orin any auxiliary equipment provided or in the electrical load network.It has been found, however, that the governor valve and the emergencyvalve, as also other valves controlling the supply of operating fluid orfuel to the prime mover sometimes fail to close, when they are intendedto close, and in such case the opening of the main circuit breakerwithout first ascertaining that the generator is not furnishing powermay give rise to dangerous overspeeding and may thus be the cause ofdestructive accidents.

The present invention has for its object to provide an improvedprotective arrangement for the prime mover, whereby risk of suchdangerous overspeeding is eliminated or minimized. In the protectivearrangement according to the present 1nvention, a blocking contact isprovided in an electrical circuit, through which crippling control meanscontrols the tripping of the main circuit-breaker, such blocking contactnormally acting to prevent the tripping control means from causing thecircuit-breaker to be tripped, and a sensitive wattmetric relayresponsive to the conditions 1n the connection between the generator andthe load acts, when the power supply from the generator to the load hasfallen below a predetermined value or has reversed, to operate theblocking contact and thereby to permit the circuit-breaker to be trippedby operation of the tripping control means. This arrangement ensuresthat the circuitbreaker will not be tripped, following operation of thetripping control means, so long as the generator is supplyingsignificant power to the load.

Means are preferably provided for giving an alarm 3,156,848 PatentedNov. 10, 1964 when tripping of the circuit-breaker has been initiated bythe tripping control means but is prevented by the blocking contact.

The invention may be carried into practice in various ways, but theaccompanying drawing illustrates by way of example a preferred practicalprotective arrangement according thereto.

In this arrangement an A.C. generator A is driven by a prime mover B,which may consist for example of a steam turbine provided in thewell-known manner with a valve, operated through a hydraulic column by agovernor responsive to the generator speed, for regulating the steamsupply to the turbine, and with an emergency valve, which is normallylatched in the open position by hydraulic pressure and whose function isto cut oil the steam supply to the turbine in the event ofover-speeding, and also with a hand-operated isolating valve, whichprovides an additional hand control of the steam supply, for use whenrequired. A mechanical shut-down knob is often provided, which can beoperated by hand in emergency to cause, by releasing the hydraulicpressure, rapid closing of the governor valve or the emergency valve orboth. During operation of the turbine over long periods of time,however, these valves are apt to stick on occasions and thereby to failto cutoff the steam supply when called upon. Such failures may arisefrom various causes, including for example jamming of the valves byforeign matter or corrosion of or chemical deposit on the valvesurfaces.

The generator A is connected to a three-phase electrical network C, orother load circuit, which it is to supply, through a maincircuit-breaker D, shown in the closed position. The circuit-breaker Dis provided in the usual manner with a trip coil D energised from anauxiliary source of power, such for example as a battery E, which isconnected to the trip coil D through an auxiliary contact D on thecircuit-breaker. The contact D is closed when the circuit-breaker isclosed. The tripping of the circuit-breaker is controlled in thewell-known manner by electric protective arrangements which areoperative automatically and may be arranged in various ways inaccordance with the circumstances of the particular installation torespond to any of a variety of abnormal conditions or faults in thegenerator A or in the steam turbine B or in associated equipment or inthe load network C. Such protective arrangements, which may causeenergisation of the trip coil D either directly or through an auxiliarycontactor, are not shown in detail in the drawing for the sake ofclearness, but are represented diagrammatically by a relay F, whosecontact F directly controls the energizing circuit of the trip coil DThe usual hand-controls are provided for closing and tripping thecircuit-breaker D during normal operation, the tripping hand controlbeing indicated at G. The contact G of this tripping hand control Gdirectly controls the energisation of the trip coil D from the source E.In addition, a further hand-operated trip button H is provided, foremergency use, the contact H of this trip button likewise controllingthe energisation of the trip coil D Such emergency trip button H may, ifdesired, be combined with the above-mentioned emergency shutdown knobcontrolling the steam supply to the turbine B, for simultaneousoperation, or may be independent thereof.

With such known arrangement, it is important to ensure that thecircuit-breaker D is not tripped when the steam supply valves to theturbine B are wide open (except deliberately for the purpose of testing,with suitable precautions), for this can give rise to dangerousoverspeeding, with disastrous eiiects, if it should happen that thesteam supply valves fail to close. The arrangement according to theinvention, now to be described, ellectively eliminates this risk, byensuring that the circuitbreaker cannot be tripped, unless the steamsupply valves are in fact closed or nearly closed, thereby making itsafe to trip the circuit-breaker.

For this purpose, a blocking contact is provided in series in thecircuit, through which the protective relay F and the emergency tripbutton H control the energisation of the trip coil D. This blockingcontact is closed only when it is safe to trip the circuit-breaker, andfor this purpose is constituted by the contact I of a sensitivewattmetric relay J, energised in any of the well-known ways inaccordance with the current and voltage conditions on the connectionbetween the generator A and the network C, for example (as shown) on thecurrent side from current transformers K on two of the phases and on thevoltage side from a three-phase voltage transformer L. The relay J isset to close its contact 5 when the power flowing out from the generatorA to the network C has fallen below a predetermined small value, or whenthe power reverses so that it is flowing into the generator from thenetwork.

It will be noticed that in cases when, owing to incorrect operation orto failure of the valves to close, the circumstances are such that itwould not be safe to trip the circuit-breaker D, the tripping of thecircuit-breaker is temporarily prevented after the tripping operationhas been initiated by operation of the emergency knob H or of theprotective relay F. Whilst in general any delay in tripping afteroperation, say, of a protective relay is un desirable, it is believedthat the risks run from such delay are much less than those which canarise from overspeeding. It should of course be made clear that the loadwill ordinarily fall oif substantially instantaneously (and thus causethe blocking contact I to close) following op eration of the shut-downknob, so that the increase in time delay will usually be insignificant.It is, however, desirable to provide an alarm device operated by analarm relay M, which is connected in parallel with the trip coil' Dexcept that it is not controlled by the blocking contact J Thus, thealarm will be given in all cases, in which the tripping of thecircuit-breaker has been initiated but is delayed by the blockingcontact J and is arranged to continue until it has been attended to bymanual shutting off of the steam supply to the turbine B.

Whilst it would be possible for the blocking contact J also to control atripping operation initiated under normal operating conditions by thetripping hand control G, this will not usually be desired and, as shownin the drawing, this hand control G controls the energisation of thetrip coil D independently of the blocking contact J controlling devices.This unlatching coil N is also in circuit with the contact G of thetripping hand control G,

so that it will not be energised when such hand control G is operated.

It has been mentioned above that the electric protective arrangementsfor the network and generator may act on the trip coil D through anauxiliary contactor, instead of directly, In such case, if the emergencytrip button H also acts through such contactor, the blocking contact Imay be in series either with the coil of the auxiliary contactor or withthe trip coil circuit controlled by the auxiliary contactor, but if theemergency trip button H acts directly on the trip coil circuit and notthrough the auxiliary contactor, the blocking contact 5 must be in thetrip coil circuit.

it will be appreciated that the foregoing arrangement has been describedby way of example and may be modified in various ways within the scopeof the invention. Thus, the arrangement is not limited to use with asteam turbine as prime mover and is applicable also (with any necessarymodifications) when other forms of prime mover are used, such forexample as gas turbines or hydraulic turbines or reciprocating steamengines or internal combustion engines. Further, although described withreference to AC. generators, it will be clear that the arrangement isequally applicable to DC. generators.

What I claim as my invention and desire to secure by Letters Patent is:

1. A protective arrangement for a prime mover driving an electricalgenerator for supplying an electrical load through a maincircuit-breaker, comprising an electrical circuit controlling thetripping of the main circuitbreaker, tripping control means acting whenoperated to control such circuit andthereby to effect tripping of thecircuit-breaker, and means exercising an over-riding control forpreventing the tripping control means from effecting tripping of thecircuit-breaker whenever the generator is supplying substantial powerthrough the circuit-breaker to the load, such over-riding control meansincluding a blocking contact in the said electrical circuit normallyrendering such circuit inoperative thereby to prevent tripping of thecircuit-breaker, and a sensitive wattmetrical relay responsive to thepower supply conditions in the electrical connection from the generatorto the load and acting, when the power supply from the generator to theload has fallen below a predetermined small valve, to release theblocking contact and thereby to permit the tripping control means toeifect tripping of the circuit-breaker.

2. A protective arrangement as claimed in claim l, in cluding means forgiving an alarm when tripping of the main circuit oreaker has beeninitiated by the tripping control means but is prevented by the blockingcontact.

References Cited in the file of this patent

1. A PROTECTIVE ARRANGEMENT FOR A PRIME MOVER DRIVING AN ELECTRICALGENERATOR FOR SUPPLYING AN ELECTRICAL LOAD THROUGH A MAINCIRCUIT-BREAKER, COMPRISING AN ELECTRICAL CIRCUIT CONTROLLING THETRIPPING OF THE MAIN CIRCUITBREAKER, TRIPPING CONTROL MEANS ACTING WHENOPERATED TO CONTROL SUCH CIRCUIT AND THEREBY TO EFFECT TRIPPING OF THECIRCUIT-BREAKER, AND MEANS EXERCISING AN OVER-RIDING CONTROL FORPREVENTING THE TRIPPING CONTROL MEANS FROM EFFECTING TRIPPING OF THECIRCUIT-BREAKER WHENEVER THE GENERATOR IS SUPPLYING SUBSTANTIAL POWERTHROUGH THE CIRCUIT-BREAKER TO THE LOAD, SUCH OVER-RIDING CONTROL MEANSINCLUDING A BLOCKING CONTACT IN THE SAID ELECTRICAL CIRCUIT NORMALLYRENDERING SUCH CIRCUIT INOPERATIVE THEREBY TO PREVENT TRIPPING OF THECIRCUIT-BREAKER, AND A SENSITIVE WATTMETRICAL RELAY RESPONSIVE TO THEPOWER SUPPLY CONDITIONS IN THE ELECTRICAL CONNECTION FROM THE GENERATORTO THE LOAD AND ACTING, WHEN THE POWER SUPPLY FROM THE GENERATOR TO THELOAD HAS FALLEN BELOW A PREDETERMINED SMALL VALUE, TO RELEASE THEBLOCKING CONTACT AND THEREBY TO PERMIT THE TRIPPING CONTROL MEANS TOEFFECT TRIPPING OF THE CIRCUIT-BREAKER.